My young lad's been having fun with the shockwave one, but I can't convince him to try out the Opensource nexuiz one... I'm not a FPS gamer, could somebody try it out to let me know if it's any good?

Being a Linux gamer, I've played Nexuiz quite a bit. Graphics are solid, as is gameplay. However, I don't find it as "tight" as say, UT2004 or QIII. It's definitely worth a go, I can say that much. LAN play works like a dream as well.